5500 to 5800 if you want it to live very long... It can probably withstand 6000 to 6200 on rare occasions, but it will self destruct if you make a habit of it. If you have it balanced and use forged or hyperuetectic pistons it can withstand 7200 to 7500 consistantly. Add a good steel crank, splayed mains, and high quality connecting rods and expect 8500+. All this is assuming the valve train will keep up...